Genetic and biochemical studies with enterobacterial reveal that nitrogen control in these organisms is quite complex. Nitrogen control can be defined as the adjustment in enzyme synthesis made by cells in response to the availability of nitrogen in the growth medium. The levels of the NH3-assimilatory enzymes, as well as some amino acid transport systems and catabolic enzymes are affected by this process. The attention of this laboratory has been directed toward elucidating the elements involved in nitrogen control and determining the degree of their functional specificity for the class of NH3-assimilatory enzymes represented by glutamine synthetase (GS), glutamate dehydrogenase (GDH), and glutamate synthase (GAT). Studies have recently focused on the possibility that an additional regulatory role in nitrogen control might exist for some of the proteins which participate in the cascade for modulation of the enzyme activity of GS.